The present invention relates to power systems. More specifically, the present invention relates to a system for managing power from multiple sources to address power demand in an elevator system.
The power demands for operating elevators range from positive, in which externally generated power (such as from a power utility) is used, to negative, in which the load in the elevator drives the motor so it produces electricity as a generator. The use of the motor to produce electricity as a generator is commonly called regeneration. In conventional systems, if the regenerated energy is not provided to another component of the elevator system or returned to the utility grid, it is dissipated through a dynamic brake resistor or other load. In this configuration, all demand remains on the power utility to supply power to the elevator system, even during peak power conditions (e.g., when more than one motor starts simultaneously or during periods of high demand). Thus, components of the elevator system that deliver power from the power utility need to be sized to accommodate peak power demand, which may by more costly and require more space. Also, the regenerated energy that is dissipated is not used, thereby decreasing the efficiency of the power system.
In addition, an elevator drive system is typically designed to operate over a specific input voltage range from a power supply. The components of the drive have voltage and current ratings that allow the drive to continuously operate while the power supply remains within the designated input voltage range. In conventional systems, when the utility voltage sags, the elevator system faults. In conventional systems, when a utility power failure occurs or under poor power quality conditions, the elevator may become stalled between floors in the elevator hoistway until the power supply returns to normal operation.
Elevator drive systems may incorporate a secondary power supply that is controlled to deliver supplemental power to the elevator hoist motor during periods of positive power demand, and store power from the power utility and/or elevator hoist motor during periods of zero or negative power demand. For example, U.S. Pat. No. 6,431,323, Tajima et al., describes an elevator drive system including a power storage apparatus and a controller for controlling charging and discharging operation of the power storage apparatus based on a charging target value (e.g., a charge value based on the time of day). However, this type of control does not provide a direct method for gauging future energy demands of the elevator drive system, and does not control the upper and lower charge limits of the power storage apparatus.